1. Field of the Invention
The present invention relates to a thin film forming apparatus used in manufacturing a display device that has an EL (electroluminescence) element (the device will hereafter be called a light emitting device), to a method of manufacturing a light emitting device using the film forming apparatus, and to a light emitting device manufactured by the method.
2. Prior Art
Researches on a light emitting device having an EL element as a self-luminous element are flourishing in recent years. The one that attracts attention most is a light emitting device using an organic material as an EL material. The light emitting device is also called an organic EL display (OELD) or an organic light emitting diode (OLED).
The light emitting device is characterized by having no viewing-angle-related problem because it is self-luminous unlike liquid crystal display devices. This makes the light emitting device a more suitable display for outdoor use than liquid crystal displays, and various application modes for the light emitting device have been proposed.
An EL element is composed of a pair of electrodes and an EL layer sandwiched between the electrodes. The EL layer usually has a laminate structure typical example of which is one proposed by Tang et al. of Eastman Kodak Company and composed of a hole transporting layer, a light emitting layer, and an electron transporting layer. This structure has so high a light emitting efficiency that it is employed in almost all of light emitting devices that are under development at present.
When a given voltage is applied to the EL layer structured as above by a pair of electrodes, recombination of carriers takes place in the light emitting layer to emit light. On one hand, there is a passive matrix method in which EL layers are placed between one kind of striped electrode and the other kind of striped electrode which intersect with each other at right angles. On the other hand, there is an active matrix method in which EL layers are placed between pixel electrodes that are connected to TFTs and arranged in matrix and opposite electrodes.
A general EL element is formed such that it emits light toward a substrate side on which TFTs are formed when viewed from the EL element side (this is called a downward (bottom face) emission type).
In the downward emission type, a pixel electrode formed on a substrate that has TFTs formed on a quartz, glass or other insulating surface (hereinafter referred to as TFT substrate) is an anode, and is formed of a light-transmissive conductive film such as an alloy film of indium oxide and tin oxide (the alloy is called ITO) or a zinc oxide film. An EL layer is formed on the pixel electrode, and formed on the EL layer is an opposite electrode that serves as a cathode and is formed of a metal material having a small work function. Patterning of the pixel electrode is required in forming the pixel electrode and, in this case, patterning by photolithography is usually employed. The opposite electrode is formed by evaporation that damages the EL layer less.